Add types declaration file

[geopic]

Resolves #1. Let me know if there are any issues.

[srebughini]

Hey,

I don't know how it works in TypeScript, but:

• GasMixtureComposition: getMassFraction, getMoleFraction and getMixtureDiffusion are 1d-array (N, where N is the number of spiecies).
• GasMixture: getMassFraction, getMoleFraction and getMixtureDiffusion are 1d-array (N, where N is the number of spiecies). And getBinaryDiffusion is a 2d-array (NxN, where N is the number of species).

Is the type number still the right one even for arrays?

[geopic]

Noted, I'll correct it now. And if the one-dimensional arrays are arrays of numbers then the correct way to label it in TS is number[] or Array<number>, both are right ways to do it